@article{10_37236_4596,
     author = {Noah Golowich and David Rolnick},
     title = {Acyclic subgraphs of planar digraphs},
     journal = {The electronic journal of combinatorics},
     year = {2015},
     volume = {22},
     number = {3},
     doi = {10.37236/4596},
     zbl = {1325.05060},
     url = {http://geodesic.mathdoc.fr/articles/10.37236/4596/}
}